At the present time, there is an increasing need for autonomous rendezvous and docking capabilities of unmanned spacecraft vehicles in order to decrease mission cost and reduce risk to human life. For instance, planned space missions call for unmanned spacecraft to take samples from other planets and planetoids and return them to earth. With respect to other unmanned missions for support for other spacecraft, support spacecraft are required to reliably “home in” and dock with another spacecraft in order for them to carry out their designed functions. At the same time, a trend is underway to assemble spacecraft and support structures in orbit to avoid requirements for heavy-lift vehicles where large payloads are required. In addition, satellite support, such as support necessary for the Hubble space telescope, may be accomplished using unmanned autonomous spacecraft, thus significantly lowering cost of maintaining satellites.
Development of autonomous rendezvous and docking sensors currently in progress include the Video Guidance Sensor and the Advanced Video Guidance Sensor, both of which are being developed at NASA's Marshall Space Flight Center in Huntsville, Ala. These systems work by illuminating with laser light several retro-reflector targets mounted to a target vehicle, optically imaging the reflected light, and processing the resultant pattern of light to determine range, bearing, and pose, or relative orientation, and provide six-degree-of-freedom information. Six degree of freedom information refers to elevation, azimuth, range, roll, pitch and yaw. Laser wavelength frequencies used in this system are 808 nm and 845 nm, with sensors on a tracking spacecraft receiving returned laser light. Here, the sensor is a camera, with processing performed on camera images to extract range and pose information. This system has a working range of roughly 500 m. NASA is currently committed to using these video guidance systems. As such, any other laser range and bearing finding system used in conjunction with the Video Guidance Sensor and Advanced Video Guidance Sensor would need to be designed so as not to interfere with these video guidance systems.
Another system is under development by OPTECH Inc., Toronto, Canada, and MD ROBOTICS, Brampton, Ontario, Canada, and is known as the RENDEZVOUS LASER VISION system. This system uses a scanning LIDAR to track spacecraft at ranges of 3.5 km, and determines pose by matching a measured 3D image to models.
As far as the Applicants are aware, relative GPS/ground based radar systems are not sufficient to allow spacecraft to dock, and do not support lunar missions.
In addition to the foregoing, systems designed for use in space need to be as simple as possible due to the harsh environment. As such, stationary systems (strap-down sensor systems) are preferable to scanning systems that require moving components. Applicants are aware of scanning laser radar systems that use a small spot of laser light to generate high return signals, but in space such systems require precise pointing of the laser, and acquisition of a target would require that the beam be scanned back and forth over a region of space where the target is expected. Such scanning requires moving parts that increase complexity, expanse and failure expectations of the system.
To these ends, Applicants propose a combined laser range and bearing finder that operates at an intermediate range out to about 5 kilometers down to about 10 meters or so, and which has a relatively wide field of view so as to allow it to operate as a strapdown sensor. It may operate as either a standalone sensor, or operate in tandem with other sensor systems that return range, bearing and orientation information at close ranges, such as the aforementioned Advanced Video Guidance Sensor.